So-called air-based air conditioning systems, as described for example in DE 10 2008 053 320 A1 or non-published DE 10 2010 054 448, are usually used at present in commercial aircraft to air-condition the aircraft cabin. An aircraft air conditioning system serves to cool the aircraft cabin, which would otherwise be overheated due to thermal loads, such as, for example, body heat of the passengers and waste heat from equipment present on board the aircraft. The aircraft air conditioning system in addition adequately supplies fresh air into the aircraft cabin to ensure that a prescribed minimum proportion of oxygen is present in the aircraft cabin.
Air-based air conditioning systems typically comprise an air conditioning unit, which is arranged, for example, in a wing root of the aircraft, and which is supplied with compressed process air that is generated by a compressor or bled of from an engine or an auxiliary power unit (APU) of the aircraft. During flight operation of the aircraft, usually engine bleed air is used so as to supply the air conditioning unit of the aircraft air conditioning system with compressed process air. During ground operation of the aircraft the air conditioning unit of the aircraft air conditioning system, however, typically is supplied with compressed process air from the auxiliary power unit of the aircraft. In the air conditioning unit, the process air, upon flowing through at least one heat exchanger as well as through various compression and expansion units, is cooled and expanded. Cooled process air exiting the air conditioning unit finally is supplied to a mixing chamber where it is mixed with recirculation recirculated from an aircraft region to be air conditioned. The mixed air from the mixing chamber, via respective mixed air lines, is supplied to the aircraft region to be air conditioned which may be divided into a plurality of air conditioning zones.